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I have regular 12-2 wire going to a bathroom fan. Tell me about code issues with just running an extra 12 gauge shielded wire up to the fan so I can do the fan and light separate. Any restrictions, is it supposed to run with the existing cable... whatever.

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I think you're asking "Can a 12/2 cable plus a single 12AWG conductor work be treated as a 12/3 cable?" I assume you're asking about using the same ground and neutral conductors. I'm pretty sure the answer is "yes" but I don't have a code reference. – Jay Bazuzi May 4 '13 at 4:38
up vote 2 down vote accepted

Doesn't look good.

National Electrical Code 2011

ARTICLE 300 Wiring Methods

300.3 Conductors.
(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

This is done to ensure that all the wires are kept close together, so that the magnetic fields involved act the way they're supposed to. And also so that the next guy can figure out what's going on, and doesn't have to search a bunch of raceways to locate all the circuit conductors.

If it was a control/signal wire; and was not supplying power for the operation of the fan, that would be a different story.

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What if they were taped and stapled together? Still a no go? – DMoore May 6 '13 at 15:12
@DMoore For that, see my favorite code section 110.12 Mechanical Execution of Work. Electrical equipment shall be installed in a neat and workmanlike manner. – Tester101 May 6 '13 at 18:39
What does neat and workmanlike manner mean? Now that is a great question. This varies widely by inspector from what I have seen. – DMoore May 7 '13 at 4:38
@DMoore It means, not like this – Tester101 May 7 '13 at 16:15
@Tester101: Given what I see when I go into most commercial buildings these days, many people don't agree with you. It's a sad commentary on today's craftspeople. – Spark May 7 '13 at 19:07

If the existing wire is the plastic Romex type then you should plan to run another 12-2 with GND wire up to the fan from the switching location. A single insulated conductor is not going to cut the safety mustard. When you run such wire then just common connect the added Neutral and GND wires into those existing connections within the boxes at each end.

It may actually (in some cases) be easier to simply remove the existing 12-2 connection and replace it with a 12-3 connection.

If your existing wiring is in conduit, flex or other shielded connection scheme according to current jurisdiction requirements then you should be adding the additional or replacement wiring in like form.

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This is just me being cheap. I have tons 12-2, 12 single strands, 14-2, and 14-3 cabling but no 12-3... So I figured why not just run a single strand of 12 next to my 12-2 to power a fan... Just never read about this in electrical code and I do everything by the book with electric. – DMoore May 4 '13 at 5:11
and damn I just realized I have a ceiling fan upstairs that is ran through a wall that I would like to throw a strand down for the fan. I am going to have to take out some drywall to run a new line if that isn't code. – DMoore May 4 '13 at 5:16
Does the fan/light need 12awg? Why not use your 14-3? – gregmac May 4 '13 at 15:56
@gregmac - He says the existing wiring is 12 AWG. Depending upon the nature of the upstream wiring back to the panel and the rating of the protection circuit breaker it may not be the safe thing to do patching in some 14 AWG wire into the down stream circuit. – Michael Karas May 4 '13 at 17:45
20 AMP breaker for existing... I know I could change it to a 15 but there is a ton of stuff on this circuit. – DMoore May 4 '13 at 18:03

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